Refactor shortcode() method

[GaryJones]

The shortcode() method currently does too much.

It:

• rebuilds the original shortcode as a string with raw attributes, for the purposes of debugging.
• parses the shortcode attributes with defaults, then sanitizes them.
• includes a short-cut hack to delete transients, used during development of the plugin.
• handles feed output.
• conditionally enqueues a style if there is HTML to show.
• debug logs the attributes and HTML.

A few of these should be split into their own logical methods.

[GaryJones]

Closing as fixed for now, since https://github.com/bradyvercher/wp-blazer-six-gist-oembed/commit/ca63f2eb0d5bfcfdf061cbdde1c26a7eade06e91 pulls out the more obvious substantial bits into new methods.